This invention relates to a method for compensating temperature on a thermal head used in thermographical recording.
In a thermal printer of the type which uses an ink ribbon and a plain paper in the prior art, a heating element on a thermal head is heated to melt ink of the ink ribbon so that the melt ink is transferred onto a recording paper to record images or the like. The amount of ink transferred onto the recording paper is controlled by the temperature of the thermal head. As shown in FIG. 1, the recorded density D of the ink has a non-linear relation against an input energy J on a thermal head and its characteristic varies depending on the ambient temperature as well as the temperature T (T.sub.1, T.sub.2, T.sub.3) of the thermal head. The slope coefficient .gamma. of the recorded density D against the input energy J is as shown in FIG. 2, of a substantially linear function against the ambient temperature and the head temperature T. The above fact is applicable to a thermal recording of the type which uses a heat sensitive paper without the ink ribbon.
The input energy J on the thermal head has a relation expressed by the following formula when a voltage applied is denoted as V, and the pulse width thereof as W. EQU J=(V).sup.2 .multidot.V (1)
Therefore, the input energy J can be controlled by changing the applied voltage V or the pulse width W. However, not only the recorded density D at a given input energy J is varied by the ambient temperature and head temperature T, but also the slope coefficient .gamma. is varied by the temperature T. Therefore, the desirable density D could not be obtained if the temperature T fluctuates even if the input energy J is minutely controlled.
When the images are recorded by a thermographical recording device using the thermal head like the above, the quality of the recorded images fluctuates widely due to the temperatur of the thermal head. In the prior art, the temperature is compensated by detecting the temperature T on the thermal head with a temperature sensor, and by controlling the input energy WD onto the thermal head based upon the detected temperature T as shown in FIG. 3. The temperature compensation can be expressed by the following equation with a, b which are parameters. EQU WD=a-b.multidot.T (2)
The thermal head may have a structure shown in FIG. 4 wherein a heating element substrate 2 is provided on a supporting plate 1, and a heating resistor 3 is provided on the heating element substrate 2. By driving an electric circuit substrate 4 provided on the supporting plate 1 to control the energy supplied on the heating resistor 3, the heating resistor 3 is heated to generate heat enough to transfer the ink from a recording medium or to record onto a recording paper. The temperature on the thermal head having the above structure is heretofore detected by a temperature sensor 5 (e.g. a thermistor) attached on the reverse surface of the supporting plate 1 or by a temperature sensor 6 on the electric circuit substrate 4 as shown in FIG. 4.
However, since the temperature sensor 5 or 6 detects the temperature ambient to the heating element substrate 2 of the thermal head in those prior art devices, the detection lags behind the actual temperature changes on the heating element substrate 2 to thereby present a wide gap between the detected temperature and the real temperature on the heating element substrate 2. Even when compensated by the above equation (2), the quality of the recorded images sometimes deteriorates because of the influence of the temperature.